NIntegrate::inumr: The integrand Cos[0.0677 n x] <<1>>[250.,x] has evaluated to non-numerical values for all sampling points in the region with boundaries {{-46.4046,-46.3046}}. >>

However, using Integrate instead of NIntegrate gave me just the input as symbols.

Neither is the coefficient.

So what am I missing? There has to be a simpler way of figurijng out the fourier coeffs of an interpolating function.
can I export data out of mathematica into .csv or some other format which is not dependent on mathematica to be interpreted?

NIntegrate can't integrate symbolic arguments. You haven't fixed n so the integrand is not numerical; fix n to something and it should work. Unfortunately cut and pasting doesn't work for the reason @Silvia gave; if you fix that maybe we can help more easily.
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aclAug 2 '12 at 20:40

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